The present invention relates to an electrophotographic imaging device using a continuous form recording sheet.
Conventionally, an electrophotographic imaging apparatus, such as, a electronic copy machine, a laser beam printer and the like, is known wherein the uniformly charged surface of a photoconductive drum is exposed to light to form a latent image, toner is adhered to the latent image to develop a toner image, and the toner image is transferred to a recording sheet and fixed thereon by a fixing unit.
In general, a laser beam printer of this kind uses a so-called cut sheet which is cut into a predetermined size as a recording sheet, and a so-called heat roll fixing unit is employed therein as a fixing unit. The heat roll fixing unit comprises a pair of fixing rollers composed of a heat roller heated to a high temperature and a backup roller (also referred to a press roller) pressed against the heat roller. The recording sheet on which a unfixed toner image is placed passes between the rollers, and then the toner is heated and pressed so that the toner image is fused and adhered on the recording sheet.
The heat roller is composed of a cylindrical roller having a heating element such as a halogen lamp or the like inserted therein and heated to a predetermined temperature. The press roller is formed of a material such as silicone rubber or the like having a heat resistant property and a predetermined elasticity and pressed against the heat roller by a predetermined pressure.
A recording sheet on which unfixed toner is placed is passed between the pair of the fixing rollers to heat the toner and press so that the toner is melted and fixed on the recording sheet. This kind of the heat roll fixing unit is advantageous in that it has an excellent heat efficiency and can be operated at a high speed.
Further, another fixing method can also be employed wherein a recording sheet is only pressed by being held between rollers pressed against the other roller so that toner is crushed by the pressure and fixed. This type of unit is known a as a pressure fixing unit.
In the heat roll fixing or pressure fixing as described above, one of the rollers, which is pressed against the other roller, is rotated and thus a recording sheet held between the pair of the fixing rollers is fed by the fixing unit. In the case of the heat roll fixing, the heat roller is usually rotated and the press roller is driven to rotate by the heat roller.
Nevertheless, in the arrangement wherein a fixing operation is carried out in such a manner that a recording sheet is held between the rollers, one of which is pressed against the other and pressed (and heated) as well as the recording sheet is fed by rotating one of the rollers, a problem arises in that a feed speed of the recording sheet is changed by the difference of a friction coefficient on the surface of the recording sheet.
More specifically, a roller (heat roller in the case of the heat roll fixing) in contact with an image carrying surface is usually driven to rotate, and the surface of the roller is finished to have a lower friction coefficient so that the toner is not adhered thereon. Therefore, the roller to be rotated may slip on the surface of the recording sheet to disturb the image and lower a feed force. Further, if the surface of the recording sheet has a very low friction coefficient, the recording sheet cannot be fed. In particular, in the case that labels or the like are discontinuously attached on a release paper, the friction coefficient of the release paper changes depending upon the portion thereof and thus the feed speed of the paper is greatly changed.
Moreover, when a continuous recording sheet or the like is used as a recording sheet and another sheet feed means is used besides the heat roll fixing unit, the recording sheet feed speeds of both feed means may not be synchronized due to the change of the feed speed as described above and thus the recording sheet is loosened or jammed.
Further, a feed speed of the recording sheet should be completely synchronized with the circumferential speed of a photoconductive drum. If the recording sheet is a continuous recording sheet or the like and no sheet feed means is provided except the fixing unit, since the feed of the recording sheet depends upon the fixing unit, an image is defectively transferred by the change of the feed speed.